Printers such as inkjet printers generally employ printheads which are mounted on a scan axis for printing in a swath across a sheet of a print medium. The print medium, whether or not of paper, may be referred to herein as a “page” for simplicity, although any print-receiving medium is encompassed by this term whether in page format, in the form of an endless web, or in the form of an article such as an envelope which is fed through the printer).
The page is incrementally advanced through the printer in a direction perpendicular to the scan axis (the direction of paper movement is known as the “media axis” or as the printing advance direction (“PAD”), and the two terms are used interchangeably herein). Between each incremental advance a swath of ink is deposited on the paper.
When an image is sent to the printer, the printing software (generally embodied in printer control circuitry) generates an image mask in which the image is split into swaths of a height equal to the height of the printhead. FIG. 1 shows a print carriage of the type used in the Hewlett-Packard Designjet 10 ps, Designjet 20 ps or Designjet 50 ps range of printers. Six print cartridges 10 are mounted on a carriage 12 which travels along a pair of parallel rails 14 defining a scan axis. The carriage is driven by a belt 16 along the scan axis. The belt is driven by a motor mounted within the printer (not shown) and a set of offboard ink reservoirs feed ink to the individual print cartridges 10 via a set of six flexible tubes (not shown) whereby each printhead can be supplied with a different coloured ink (e.g. dark cyan, light cyan, dark magenta, light magenta, yellow and black).
Each print cartridge 10 supplies ink to a printhead or pen 18 comprising a linear array of 300 nozzles (arranged in two parallel staggered rows 20,22 of 150 nozzles each) running in the direction of the media axis. The nozzles in each pen are spaced at intervals of {fraction (1/600)} of an inch along the media axis, and the pens are spaced apart from one another along the scan axis.
In use the printer software converts images to be printed into an image mask of pixels of the six different colours. High quality colour hues can be printed by an appropriate mix of coloured dots laid alongside or on top of one another. In a 600 dpi (dot per inch) print mode, therefore, each square inch of the image will be pixelated into a 600×600 grid, and each point of the paper will either be left blank or will receive a droplet of ink from one or more of the pens. The manner in which the droplets are laid down is specified in the print mode.
In a low quality but high speed single pass print mode, the image mask is divided into a series of swaths running parallel to the scan axis, each swath having a ½ inch height (assuming 600 dpi quality and a pen comprising 300 active nozzles at a spacing of {fraction (1/600)} of an inch). The paper advances in ½ inch steps in the printing advance direction (PAD). As each successive ½ inch swath of paper is located under the path of the print carriage, the carriage scans across the scan axis and the individual nozzles within the pens fire in a timed sequence under the control of the printer control circuitry to deposit drops of the relevant coloured inks onto the paper in the positions called for by the image mask.
Thus, whenever a drop of dark cyan ink is specified in this strip, the appropriate nozzle is caused to fire as it passes over that point in the page. The entire ½ inch swath receives its full image in a single pass and the page is then advanced by ½ inch before the adjoining swath is printed.
However, if the paper advance mechanism is not accurate in advancing the paper by ½ inch exactly, there will be either a gap between successive swaths or an overprinting of the bottom rows of one swath and the top rows of the next swath. Generally these errors will be periodic.
Because the darkness or colour saturation between adjacent swaths is not exactly matched another element of banding occurs at the swath interfaces. Ink deposited along the swath boundaries is deposited on a different wetness gradient than ink in the regions between the lines. While this effect is not very pronounced in high pass print modes, there is nevertheless the potential for colour mismatch at the swath boundaries.
Such effects are collectively known as banding errors as they result in printing errors located in periodically repeating bands down the page. The present invention is directed, in part, to reducing such errors.
A number of solutions to address the problems of banding errors already exist and these can reduce the visibility of the banding errors to a greater or lesser extent. For example, higher quality printing is normally carried out in multiple passes, partly to reduce the visibility of banding errors and partly to deposit less ink per swath, so that the paper does not become saturated with ink (which can cause the paper to warp). In such modes, the end nozzles of the pens print at more frequent intervals down the page (e.g. in the case of a ½ inch printhead the end nozzles would deposit ink every ¼ inch in a two-pass print mode or every {fraction (1/16)} inch in an eight-pass print mode).
While this increases the frequency at which banding errors occur (since there are now e.g. twice as many bands or eight times as many bands), in fact it reduces the severity of each band, since a pixel lying at the top or bottom of a swath can be printed by either the nozzles at the top or bottom of the pen, or by the nozzles towards the middle of the pen. In a two-pass mode, each pixel has a 50% chance of being printed by either of two nozzles separated by a ¼ inch gap. In an eight-pass mode, the same pixel has a 12.5% chance of being printed by any one of 8 nozzles located at {fraction (1/16)} inch intervals along the pen.
For these reasons, the droplets of ink deposited along the swath boundaries in multiple-pass modes are mixed with droplets printed from the middle of the pens during different passes, so that the visibility of the swath boundaries decreases.
Another approach to reducing swath boundaries is to stagger the individual pens relative to one another. This means that the swath boundaries from e.g. the cyan pen in a four-colour printer will be offset from the swath boundaries of the yellow, magenta and black pens (each of which are staggered from one another also). However, in printers used for high quality printing the print cartridges for each colour are separately mounted on a carriage (and thus a four colour carriage will have four printhead units located on it). Staggering the printheads on the carriage causes problems of balance as the carriage scans, particularly at high scan speeds, thus causing a degradation of image quality
Another existing solution is to decrease the printing density from the ends of the printheads so that the amount of ink ejected from the end nozzles is relatively less than from the more central nozzles. The printing software defining the print mode causes the more central nozzles to deposit relatively more ink than the end nozzles in multiple-pass print modes and thereby reduce the ink density attributable to the swaths. In other words, the amount of ink which issues from the end nozzles during a print job is relatively less than would otherwise be expected, and the amount issuing from the more central nozzles relatively more, with the central nozzles compensating for the end nozzles during the multiple passes. However, this solution does not work for single pass print modes since there will be a necessity to print all of the image at certain positions on the page from the end nozzles only and thus the amount of ink issuing from these nozzles cannot be decreased. In most printers there is a need for both single-pass and multiple-pass print modes.